Srange Prodigy problem

Hi all,
I have a very strange problem with my Tekonsha Prodigy. When I am stationary, I can wind up the voltage all the way to 13 with no worries. However, as soon as I am mobile and use either the lever or the brake pedal, it goes up to around 1 volt then reports an overload. It then pulses in and out of overload as I move. Speed is not a factor - i can be just crawling and it happens. Even stranger is that it doesn't play up in reverse and works perfectly driving backwards.

Before you suggest I have a problem with worn magnets, they are all OK. I have just finished pulling everything apart and no sign of any problem either with the magnets or with any wiring. I put 12 volts directly on to the plug and wheels lock as they should with no sign of any overload condition.

I am starting to think it is the controller itself, but thought I would run this past the forum before going too much further. Has anyone experienced this issue before?

My next step is to borrow a controller and try that out.

Unfortunately (?) I am now off to the pub for tea, but will be back later to check, so sorry in advance if I don't respond immediately to any questions.

Cheers
Gerry